Summary

Following a short historical survey an absolute definition of seismicity based on seismic energy is proposed, the seismic energy being determined by the magnitude. However, as the values of magnitude are not available for previous observations, it is suggested to use, as relative measure of seismicity, the product of intensity and extension of the earthquake region showing good invariance properties and easily calculable for any earthquake. At last some methods are discussed for representing objectively sources of seismic energy, frequency of earthquakes, and regions exposed to danger of earthquakes.